Optical waveguide element, optical modulator using same, and optical transmission device

Abstract

The purpose of the present invention is to provide an optical waveguide element with which a fine optical waveguide can be easily manufactured and which suppresses optical conversion loss.　An optical waveguide element according to the present invention comprises: an optical waveguide substrate that has an optical waveguide formed thereon; a support substrate that supports the optical waveguide substrate; and an intermediate layer that is arranged between the optical waveguide substrate and the support substrate. Said optical waveguide element is characterized in that at least one end of the optical waveguide includes a first layer (LA1) and a second layer (LA2) that is arranged on the first layer and has a narrower width than the first layer, both of the tip end section of the first layer and the tip end section of the second layer have tapered shapes that narrow toward the tip, and the center line C1 of the tip end section of the first layer and the center line C2 of the tip end section of the second layer which are aligned in the extension direction of the optical waveguide at the one end of the optical waveguide are arranged so as to be offset from each other in the plan view of the optical waveguide substrate.

Description

Optical waveguide element, optical modulator using the same, and optical transmitter

[0001] The present invention relates to an optical waveguide element, an optical modulator using the same, and an optical transmitter, and more particularly to an optical waveguide element including an optical waveguide substrate on which an optical waveguide is formed, a support substrate that supports the optical waveguide substrate, and an intermediate layer disposed between the optical waveguide substrate and the support substrate, the optical waveguide element including a spot size conversion section, and an optical modulator and an optical transmitter using the same.

[0002] In recent years, with the increase in the amount of information, there has been a demand for faster and larger capacity optical communications for long-distance, intercity, and datacenter use. Furthermore, due to the space limitations of base stations, it has become necessary to increase the speed and miniaturize optical modulators. To miniaturize optical modulators, miniaturization of optical waveguides is effective, as strengthening the light confinement effect reduces the bending radius of the optical waveguide, making miniaturization possible.

[0003] LiNbO with electro-optic effect ３ (hereinafter referred to as LN) has been used as an optical modulator for long distances because it has little distortion and low optical loss when converting electrical signals to optical signals, but conventional optical waveguides have a mode field diameter (MFD) of about 10 μmφ and a large bending radius of several tens of mmφ, making it difficult to miniaturize them. Currently, advances in substrate bonding and smart cut technology have made it possible to thin LN substrates, and research and development of LN optical waveguides with a diameter of about 1 μm is progressing.

[0004] On the other hand, in an optical waveguide element including a fine optical waveguide 10 with a small MFD, when light is incident (L1) or emitted (L2) from the element end face, if an optical fiber is directly bonded to the element end face, a large insertion loss occurs. For this reason, the use of an electron beam lithography system capable of fine patterning to form a narrow linewidth LN optical waveguide and provide a spot size converter (SSC) within the chip has been considered. However, electron beam lithography systems have long lithography times, and there are limitations on the fine linewidth depending on the thickness of the resist mask. Furthermore, the alignment tolerance is narrow, which can lead to increased conversion loss due to misalignment.

[0005] In Patent Document 1, as shown in Fig. 2, a spot size converter is formed by forming a fine second-stage LN waveguide 11 on a tapered first-stage LN waveguide 12, and coating the entire structure with an insulating film 3 to convert light in multiple stages, thereby forming an SSC. However, because the second-stage optical waveguide 11 needs to have a fine shape, electron beam lithography requires a long working time, while photoresist has a limit to the fine line width, resulting in increased conversion loss.

[0006] In Patent Document 2, when forming the core 102 and auxiliary cladding 101, a resist is patterned into a tapered shape by photolithography, and then etching is performed to form the core 102 into a tapered shape, thereby forming a thin wire portion. Note that an upper cladding 300 is disposed above the optical waveguide 100 formed by the core 102 and auxiliary cladding 101, and a lower cladding 200 and a substrate 400 are disposed below it. However, although this structure can suppress optical loss because light spreads slowly in the lateral direction, light in the vertical direction (thickness direction of the optical waveguide) is rapidly converted, resulting in large conversion loss.

[0007] JP 2022-56980A JP 2011-242650A

[0008] The object of the present invention is to solve the above-mentioned problems, to provide an optical waveguide element that can easily manufacture a fine optical waveguide and suppresses optical conversion loss, and to provide an optical modulator and an optical transmitter that use the optical waveguide element.

[0009] In order to solve the above problems, the optical waveguide element of the present invention, and the optical modulator and optical transmitter using the same have the following technical features: (1) An optical waveguide element including an optical waveguide substrate on which an optical waveguide is formed, a support substrate that supports the optical waveguide substrate, and an intermediate layer disposed between the optical waveguide substrate and the support substrate, wherein at least one end of the optical waveguide has a first layer and a second layer that is disposed on the first layer and has a width narrower than that of the first layer, and both of the tip ends of the first layer and the second layer have tapered shapes that narrow toward the end, and the center lines of the tip ends of the first layer and the second layer that are along the extension direction of the optical waveguide at the one end of the optical waveguide are misaligned from each other when the optical waveguide substrate is viewed in plan.

[0010] (2) In the optical waveguide element described in (1) above, one edge of the tapered shape of the second layer overlaps a part of one edge of the tapered shape of the first layer.

[0011] (3) In the optical waveguide element described in (1) or (2) above, the tip of the first layer and the tip of the second layer are both covered with a material having a refractive index lower than the refractive index of the optical waveguide substrate.

[0012] (4) In the optical waveguide element according to any one of (1) to (3) above, the first layer and the second layer are formed of the same material.

[0013] (5) In the optical waveguide element according to any one of (1) to (4) above, at least one of the tip of the first layer and the tip of the second layer is characterized in that the thickness of the tip becomes thinner toward the tip of the tapered shape.

[0014] (6) In the optical waveguide element according to any one of (1) to (5) above, the optical waveguide is a rib-type optical waveguide having a rib portion, and the rib portion is formed at the same position in the thickness direction as the second layer of the optical waveguide substrate.

[0015] (7) In the optical waveguide element according to any one of (1) to (6) above, the intermediate layer is formed of a material having a refractive index lower than that of the optical waveguide substrate, and the intermediate layer has a step portion in accordance with the tapered shape of the first layer.

[0016] (8) In the optical waveguide element described in any one of (1) to (7) above, the tip of the second layer has a bent portion that bends toward the tapered edge of the first layer.

[0017] (9) An optical modulator comprising: an optical waveguide element according to any one of (1) to (8) above; a housing for accommodating the optical waveguide element; and an optical fiber for inputting or outputting a light wave to or from the optical waveguide.

[0018] (10) In the optical modulator described in (9) above, the optical waveguide element is characterized in that it has a modulation electrode for modulating a light wave propagating through the optical waveguide, and has an electronic circuit inside the housing for amplifying a modulation signal input to the modulation electrode.

[0019] (11) An optical transmitter comprising: an optical modulator according to (9) or (10) above; a light source for inputting a light wave to the optical modulator; and an electronic circuit for inputting a modulation signal for modulating the light wave to the optical modulator.

[0020] The present invention provides an optical waveguide element comprising an optical waveguide substrate having an optical waveguide formed thereon, a support substrate supporting the optical waveguide substrate, and an intermediate layer disposed between the optical waveguide substrate and the support substrate, wherein at least one end of the optical waveguide has a first layer and a second layer disposed on the first layer and narrower than the first layer, the end portions of the first layer and the second layer both having tapered shapes, and the center lines of the end portions of the first layer and the second layer along the extension direction of the optical waveguide at the one end of the optical waveguide are offset from each other in a plan view of the optical waveguide substrate, thereby enabling the easy manufacture of fine optical waveguides and providing an optical waveguide element with reduced optical conversion loss. Furthermore, by using an optical waveguide element with such excellent characteristics, it is possible to provide an optical modulator and an optical transmitter that achieve similar effects.

[0021] 9 is a plan view showing an outline of an optical waveguide element to which the present invention is applied. It is a perspective view showing an example of a conventional optical waveguide element described in Patent Document 1. It is a perspective view showing an example of a conventional optical waveguide element described in Patent Document 2. It is a plan view (viewed from above) explaining an optical waveguide element of the present invention. It is a cross-sectional view taken along dash-dotted lines A to C in FIG. 4. It is a cross-sectional view taken along dash-dotted lines D to F in FIG. 4. It is a cross-sectional view taken along dash-dotted line A in FIG. 4. It is a cross-sectional view taken along dash-dotted line C in FIG. 4. It is a diagram explaining a procedure for manufacturing an optical waveguide element of the present invention. It is a cross-sectional view taken along dash-dotted line X in STEP 6. It is a cross-sectional view taken along dash-dotted line X and Y in STEP 6. It is a graph showing the results of a simulation of optical conversion loss in an optical waveguide element of the present invention. It is a diagram explaining the principle of suppressing optical conversion loss in an optical waveguide element of the present invention. It is a plan view explaining the shape of one end of an optical waveguide in an optical waveguide element of the present invention. It is a diagram explaining the problem of misalignment of a resist pattern. 1 is a diagram illustrating an example of an application of an optical waveguide element according to the present invention;

[0022] The optical waveguide element of the present invention, and the optical modulator and optical transmitter using the same will be described in detail below using preferred examples. An example of the optical waveguide element of the present invention is shown in plan views in Figures 1 and 4 and in cross-sectional views in Figures 5 and 6. The optical waveguide element of the present invention is an optical waveguide element comprising an optical waveguide substrate 1 on which an optical waveguide 10 is formed, a support substrate 4 that supports the optical waveguide substrate 1, and an intermediate layer 2 that is arranged between the optical waveguide substrate 1 and the support substrate 4, wherein at least one end of the optical waveguide has a first layer (LA1) and a second layer (LA2) that is arranged on the first layer and has a width narrower than that of the first layer, and the tip end portions of the first layer and the second layer both have tapered shapes that narrow toward the end, and a center line C1 of the tip end portion of the first layer and a center line C2 of the tip end portion of the second layer that are along the extension direction of the optical waveguide at the one end of the optical waveguide are arranged to be offset from each other when the optical waveguide substrate is viewed in a plane.

[0023] The substrate (optical waveguide substrate) 1 used in the optical waveguide element of the present invention can be a substrate having an electro-optic effect. Specifically, substrates such as lithium niobate (LN), lithium tantalate (LT), and PLZT (lead lanthanum zirconate titanate), as well as substrates made of these substrate materials doped with MgO or the like, can be used. These materials can also be used to form films using vapor phase growth methods such as sputtering, evaporation, or CVD. Furthermore, a substrate made by bonding a substrate having an electro-optic effect to another substrate and then thin-film processing the electro-optic substrate can also be used. Furthermore, semiconductor substrates and substrates made of organic materials such as EO polymers can also be used.

[0024] The optical waveguide 10 can be an optical waveguide in which a high refractive index material such as Ti is thermally diffused into a substrate (optical waveguide substrate) 1, an optical waveguide formed by proton exchange, or a rib-type optical waveguide 10 in which the portion of the substrate corresponding to the optical waveguide is convex (rib portion) as shown in Figure 2, by etching the substrate 1 other than the optical waveguide or by forming grooves on both sides of the optical waveguide. Furthermore, in accordance with the rib-type optical waveguide, it is also possible to further increase the refractive index by diffusing Ti or the like onto the substrate surface by thermal diffusion or proton exchange. The size of the rib-type optical waveguide is an optical waveguide with a fine structure with a width and height of about 1 μm to enhance light confinement.

[0025] The thickness (maximum thickness) of the optical waveguide substrate (thin plate) 1 on which the optical waveguide 10 is formed is set to 10 μm or less, more preferably 5 μm or less, and even more preferably 1 μm or less, in order to achieve speed matching between the microwave and light waves of the modulation signal. Furthermore, the height of the rib-type optical waveguide 10 (the height of the portion protruding from the slab waveguide) is set to 80% or less of the maximum thickness of the optical waveguide substrate, specifically 4 μm or less, more preferably 3 μm or less, and even more preferably 0.8 μm or less or 0.4 μm or less.

[0026] In order to increase the mechanical strength of the optical waveguide substrate 1 on which the optical waveguide is formed, a support substrate 4 is bonded to the underside of the optical waveguide substrate 1. The optical waveguide substrate 1 and the support substrate 4 are bonded and fixed via an intermediate layer 2. The intermediate layer 2 is made of a material having a refractive index lower than that of the optical waveguide substrate 1. Specifically, SiO ２ , Al ２ O ３ A metal oxide such as SiO 2 or the like is used for the intermediate layer. Furthermore, the support substrate 4 is preferably made of a material having a thermal expansion coefficient close to that of the optical waveguide substrate 1, for example, a substrate including an oxide layer of quartz, glass, or the like. Furthermore, it is also possible to use the same LN substrate as the optical waveguide substrate 1, or a composite substrate in which a silicon oxide layer is formed on a silicon substrate, abbreviated as SOI or LNOI, or a composite substrate in which a silicon oxide layer is formed on an LN substrate. Furthermore, the support substrate according to the present invention is not limited to one formed of a single substrate, but also includes a substrate formed by stacking multiple substrates together.

[0027] The configuration of the spot size converter provided at one end of the optical waveguide, which is a feature of the optical waveguide element of the present invention, will be described below. Fig. 4 is an enlarged plan view of one end of the optical waveguide, and Figs. 5A to 5C and Fig. 6D to 6F show cross-sectional views taken along dashed lines A to F in Fig. 4.

[0028] As shown in FIGS. 4 and 5A, one end of the optical waveguide 10 constituting the spot size conversion section is provided with a first layer (LA1) and a second layer (LA2) disposed on the first layer and narrower than the first layer. When the optical waveguide substrate 1 is formed by stacking multiple different materials, the first layer (LA1) and the second layer (LA2) may be made of different materials. Alternatively, the optical waveguide substrate may be made of a single material, such as an LN substrate, and the first layer (LA1) and the second layer (LA2) may be formed by partially varying the etching depth of the optical waveguide substrate 1. By constructing the first layer and the second layer from the same material, it is possible to further suppress optical conversion loss when the MFD expands from the second layer to the first layer. Furthermore, a portion (ML1) of the intermediate layer 2 disposed below the first layer (LA1) may be incorporated as part of the first layer.

[0029] In the optical waveguide element of the present invention, the tip end portion of the first layer (LA1) and the tip end portion of the second layer (LA2) both have a tapered shape, as shown in Fig. 4. Moreover, as shown in Fig. 4, which is a plan view of the optical waveguide element, the center line C1 of the tip end portion of the first layer (LA1) and the center line C2 of the tip end portion of the second layer (LA2) along the extension direction of the optical waveguide at one end (the vertical direction in the drawing) are arranged to be offset from each other.

[0030] In the conventional example of Fig. 2, two tapered tip portions are also arranged to overlap, but the center lines of the lower tip portion and the upper tip portion are arranged to overlap when the optical waveguide element is viewed in plan view. For this reason, the optical waveguide element of Fig. 4 can be said to have a shape that is completely different from the conventional example of Fig. 2.

[0031] The following two effects can be expected from the configuration in which the center line C1 of the tip portion of the first layer (LA1) and the center line C2 of the tip portion of the second layer (LA2) are offset from each other. (1) The resist patterns used to form the tip portions of each layer overlap each other, making it possible to form a finer tapered shape for the second layer (LA2), for example, as shown in FIG. 6D. (2) Because the center lines of the two tip portions are offset as shown in FIG. 4, as a result, one edge of the tapered shape of the second layer (LA2) is positioned close to a portion of one edge of the tapered shape of the first layer (LA1), as shown in FIGS. 5B, 5C, and 6D. Alternatively, one edge of the tapered shape of the second layer (LA2) is positioned so as to overlap a portion of one edge of the tapered shape of the first layer (LA1). Therefore, as described below, the direction in which the MFD of the light wave changes is limited, making it possible to suppress optical conversion loss.

[0032] The two layers (LA1, LA2) constituting the spot size conversion section are continuously connected to the optical waveguide 10 described on the optical waveguide substrate 1, as shown in Fig. 2. For example, as shown in Fig. 2, it is possible to configure the second layer (upper tip 11) to be connected to the rib section of the optical waveguide, and the first layer (lower tip 12) to be connected to the slab waveguide located below the rib-type optical waveguide 10. Naturally, it is also possible to configure the height of the rib section of the optical waveguide 10 to be smaller or larger than the thickness of the second layer (LA2).

[0033] In the spot size conversion section, the tip of the first layer (LA1) and the tip of the second layer (LA2) are both covered with a material having a refractive index lower than that of the optical waveguide substrate. This covering layer 3 functions as a cladding portion of the optical waveguide having the tip as a core portion where the width of each tip is wide. As the width of the tip becomes narrower, the covering layer 3 functions as a core portion of the optical waveguide. If necessary, a layer with a lower refractive index can be provided to cover the covering layer 3. By having this covering layer 3, it is possible to expand the MFD from 1.0 μm to 3.0 μm, for example.

[0034] The material of the coating layer 3 is SiO ２、 Al２ O ３ The covering layer 3 can be made of an inorganic material such as SiO 2 or a resin material such as a photoresist (permanent resist). The covering layer 3 can also be used as a protective film that covers the optical waveguide 10 other than the spot size conversion portion.

[0035] Next, the edge angles of the first layer and the second layer will be described. FIG. 7 is an enlarged view of a portion of the cross-sectional view taken along dashed line A in FIG. 4, and FIG. 8 is an enlarged view of a portion of the cross-sectional view taken along dashed line C in FIG. 4. The shape shown in FIG. 7 appears in region AR1 of FIG. 4, and the shape shown in FIG. 8 appears in region AR2 of FIG. 4. As will be described later, the shape of each layer is etched using a photoresist with a corresponding pattern, so the first layer is etched more frequently than the second layer. Therefore, the edge angles θ1 and θ2 of the first layer are smaller than the edge angle θ3 of the second layer.

[0036] The smaller the edge angles θ1 and θ2 of the first layer are, the smaller the cross-sectional area of ​​the first layer becomes, and the light waves confined within the first layer gradually spread into the coating layer 3, resulting in smaller optical conversion losses in the spot size conversion section.

[0037] On the other hand, as shown in Figure 8, in the area where the edge of the first layer (LA1) and the edge of the second layer (LA2) overlap, they are formed simultaneously by a single etching process, so the edge angles θ2' and θ3' (the same angle as θ3) of each layer are the same angle and are larger than the other edge angle θ1 of the first layer. By making the edge surfaces of the first and second layers flush with each other and increasing the edge angles (θ2' and θ3'), the light waves confined within each layer are less likely to spread into the coating layer 3. This makes it possible to control the region where the MFD of the light wave changes, contributing to the miniaturization of the spot size conversion unit.

[0038] The manufacturing process of the first layer and the second layer will be described with reference to Figures 9 to 11. Figure 9 is a plan view of the optical waveguide element, and Figures 10 and 11 are cross-sectional views of each step in Figure 9. The cross-sections are located at the position of dashed dotted line X shown in STEP 6 in Figure 9 in Figure 10, and at the positions of dashed dotted lines X and Y shown in STEP 6 in Figure 11.

[0039] (STEP 1) Prepare an optical waveguide substrate 1. As shown in Fig. 10, an intermediate layer 2 is provided below the optical waveguide substrate 1, and a support substrate is omitted here.

[0040] (STEP 2) Photoresist PR1 is placed on the upper surface of the optical waveguide substrate 1. The shape of the photoresist PR1 corresponds to the shape of the first layer (LA1).

[0041] (STEP 3) Etching is performed using the pattern of the photoresist PR1. The etching may be dry etching or wet etching, and the etchant and other materials are appropriately selected depending on the material of the optical waveguide substrate. Because the optical waveguide substrate 1 is under-etched below the photoresist PR1, the upper surface of the optical waveguide substrate 1 penetrates inside the photoresist PR1. This forms the edge surface with the edge angle θ3 in FIG. 7 and the edge surfaces (edge ​​angles θ2', θ3') formed by a single etching in FIG. 8. Furthermore, etching is performed until at least the optical waveguide substrate 1 is removed, and etching may be performed down to the intermediate layer 2 if necessary.

[0042] (STEP 4) After etching is completed, the photoresist PR1 is removed.

[0043] (STEP 5) Next, a photoresist PR2 having a pattern corresponding to the second layer (LA2) is disposed. Fig. 11(X) is a cross-sectional view taken along the dashed line X in Fig. 9, and Fig. 11(Y) is a cross-sectional view taken along the dashed line Y in Fig. 9.

[0044] (STEP 6) The optical waveguide substrate 1 is etched according to the pattern of the photoresist PR2. At this time, etching is performed so that the thickness of the optical waveguide substrate 1 becomes the thickness of the first layer (LA1). During this etching, the intermediate layer 2 is also etched, and an intermediate layer (ML2) is formed in the areas where there is no photoresist PR2, and an intermediate layer (ML1') is formed in the areas where there is photoresist PR2. One edge of the tapered shape of the second layer (LA2) is formed by the etching in STEP 3, and the other edge is formed by the etching in STEP 6. This makes it possible to form a fine tip portion of the second layer.

[0045] (STEP 7) After etching is completed, the photoresist PR2 is removed.

[0046] As described above, in the manufacturing process for the optical waveguide element of the present invention, the tapered shape of the second layer (LA2) is formed using two photoresist patterns, which makes it easy to perform fine processing. Even if the positions of the two photoresist patterns are misaligned, the second layer can be reliably formed, which increases the tolerance for misalignment during manufacturing.

[0047] Next, the results of a simulation of the effect of reducing the optical conversion loss of the optical waveguide element of the present invention are shown. The center line C1 of the first layer (LA1) and the center line C2 of the second layer (LA2), shown by the two-dot chain lines in Figure 4, were set, and the optical conversion loss (ΔLoss) was calculated when the distance S (shift amount) between the center lines was changed. When the distance S was 0 μm, the two center lines were aligned. The results of further changing the distance S to 0.5 μm and 1.0 μm are shown in the graph of Figure 12.

[0048] As the distance S (shift amount) increases, the optical conversion loss (ΔLoss) decreases. The reason for this is that, as shown in FIG. 13, in the case of FIG. 13A where the second layer (LA2) is located inside the first layer (LA1), the light wave LW spreads over a wide range as indicated by the dotted arrow, resulting in increased optical conversion loss. In contrast, as shown in FIG. 13B, when the second layer (LA2) is located closer to one edge of the first layer (LA1) (asymmetric spot size conversion section SSC), the spread of the light wave LW is suppressed, resulting in reduced optical conversion loss. As mentioned above, the edge angle θ3 (θ2', θ3') shown in FIGS. 7 and 8 also contributes to suppressing the increase in the MFD of the light wave.

[0049] The position of the tip of the second layer affects the suppression of optical conversion loss. Therefore, as shown in FIG. 14 , we consider the distance D1 from the tip of the tip of the second layer to the tip of the tip of the first layer and the width W of the first layer at the tip position of the tip of the second layer. Simulation results show that a position where the distance D1 is 80 μm or more and the width W is 5 μm or less is preferable. The relationship between the width W and the distance D1 depends on the angle θ4 of the tip of the tip of the first layer. Therefore, θ4 should be 8 degrees or less, preferably 4 degrees or less, and more preferably 2 degrees or less. Generally, the smaller θ4 is, the wider the range in which the two conditions of width W and distance D1 are satisfied.

[0050] Next, we will explain a method for reducing the effect of misalignment of photoresist patterns in the manufacturing process of optical waveguide elements. As shown in Figure 15, (a) shows an example in which the photoresist patterns are arranged in a normal state. In (b), the photoresist pattern forming the second layer (LA2) is misaligned, and one edge of the second layer cannot be formed using the photoresist pattern of the first layer.

[0051] To resolve this problem, a bent portion (point Z, angle θ5) is formed in the second layer (LA2) as shown in FIG. 16, providing a structure in which the tip of the optical waveguide is bent. PR indicates the shape of the resist pattern used to form the second layer. The presence of such a bent portion ensures that the waveguide in the second layer can reliably form a thin-line structure, even if misalignment occurs, as shown in (c) or (d) of FIG. 15. To suppress optical conversion loss, the distance D2 from the bent portion to the tip of the first layer (LA1) is preferably 130 μm or greater, more preferably 210 μm or greater, and even more preferably 300 μm or greater. Furthermore, the angle θ5 of the bent portion is preferably 45 degrees or greater.

[0052] In the above explanation, the thickness of the first layer and the second layer is constant, but it is also possible to configure each of the layers so that the thickness gradually decreases toward the tip of each of the tip portions. In this case, it is possible to further reduce the optical conversion loss.

[0053] Furthermore, in the optical waveguide element of the present invention, a step is formed between the first layer and the second layer. This allows for increased adhesion between the optical waveguide substrate 1 and the covering layer 3 that covers these layers. Furthermore, a protrusion ML1' is formed on the intermediate layer at a location separated from the first layer, as shown in Figure 6(F). The presence of this protrusion ML1' also contributes to increasing adhesion between the covering layer 3 and the intermediate layer.

[0054] Next, examples of application of the optical waveguide element of the present invention to an optical modulator or optical transmitter will be described. While the following description will use an example of HB-CDM, the present invention is not limited to this, and can also be applied to optical phase modulators, optical modulators with polarization combining functions, optical modulators integrating more or fewer Mach-Zehnder type optical waveguides, bonding devices with optical waveguide substrates made of other materials such as silicon, devices for sensor applications, and the like.

[0055] As shown in FIG. 17 , the optical waveguide element includes an optical waveguide 10 formed on an optical waveguide substrate 1 and electrodes (not shown), such as a modulation electrode, that modulate the light waves propagating through the optical waveguide 10. The substrate 1 is housed within a housing CA. Furthermore, an optical modulator MD can be configured by providing an optical fiber (F) that inputs and outputs light waves to the optical waveguide. In FIG. 17 , the optical fiber (F) is introduced into the housing CA through a through-hole penetrating the sidewall, and the optical waveguide substrate 1 and the optical fiber are directly bonded. Alternatively, the light wave L1 incident from the optical fiber F and the light wave L2 emitted from the optical fiber F can be optically coupled to the optical waveguide 10 within the optical waveguide substrate 1 via an optical block equipped with an optical lens, a lens barrel, a polarization multiplexer, or the like. Furthermore, to ensure stable bonding with the optical fiber or optical block, a reinforcing member RI can be placed on top of the optical waveguide substrate 1 along the end face of the substrate 1.

[0056] An optical transmitter OTA can be configured by connecting an electronic circuit (digital signal processor, DSP) that outputs a modulation signal S0 that causes the optical modulator MD to perform modulation operations to the optical modulator MD. To obtain the modulation signal S to be applied to the optical waveguide element, the modulation signal S0 output from the digital signal processor DSP can be amplified. For this reason, in FIG. 17, a driver circuit DRV is used to amplify the modulation signal. The driver circuit DRV and digital signal processor DSP can be located outside the housing CA, but they can also be located inside the housing CA. In particular, locating the driver circuit DRV inside the housing can further reduce the propagation loss of the modulation signal from the driver circuit. If degradation of the modulation signal is minimal, a DRV is not necessary, and the optical modulator MD can be directly modulated by the DSP.

[0057] The input light L1 to the optical modulator MD may be supplied from outside the optical transmitter OTA, but it may also be incorporated integrally into the optical transmitter OTA using a semiconductor laser (not shown) as a light source. The output light L2 modulated by the optical modulator MD is output to the outside via an optical fiber F.

[0058] As described above, according to the present invention, it is possible to easily manufacture a fine optical waveguide, and to provide an optical waveguide element with reduced optical conversion loss. Furthermore, it is possible to provide an optical modulator and an optical transmitter using the optical waveguide element.

[0059] REFERENCE SIGNS LIST 1 Optical waveguide substrate (thin plate, film body) 2, ML1, ML2, ML1' Intermediate layer 3 Covering layer 4 Support substrate 10 Optical waveguide LA1 First layer (optical waveguide substrate) LA2 Second layer (optical waveguide substrate) RI Reinforcing member F Optical fiber CA Housing MD Optical modulator DRV Driver circuit DSP Digital signal processor OTA Optical transmitter

Claims

1. An optical waveguide element comprising an optical waveguide substrate on which an optical waveguide is formed, a support substrate that supports the optical waveguide substrate, and an intermediate layer disposed between the optical waveguide substrate and the support substrate, wherein at least one end of the optical waveguide has a first layer and a second layer that is disposed on the first layer and has a width narrower than that of the first layer, and the tip ends of the first layer and the second layer both have tapered shapes that narrow toward the end, and the center lines of the tip ends of the first layer and the second layer that are along the extension direction of the optical waveguide at said one end of the optical waveguide are arranged to be offset from each other when the optical waveguide substrate is viewed in a plane.

2. An optical waveguide element according to claim 1, wherein one edge of the tapered shape of said second layer overlaps a portion of one edge of the tapered shape of said first layer.

3. An optical waveguide element according to claim 1, characterized in that the tip of the first layer and the tip of the second layer are both covered with a material having a refractive index lower than that of the optical waveguide substrate.

4. An optical waveguide element according to claim 1, wherein the first layer and the second layer are formed from the same material.

5. An optical waveguide element according to claim 1, characterized in that at least one of the tip of the first layer and the tip of the second layer has a tapered shape with a reduced thickness toward the tip.

6. An optical waveguide element according to claim 1, wherein the optical waveguide is a rib-type optical waveguide having a rib portion, and the rib portion is formed at the same position in the thickness direction as the second layer of the optical waveguide substrate.

7. An optical waveguide element according to claim 1, wherein the intermediate layer is formed of a material having a refractive index lower than that of the optical waveguide substrate, and the intermediate layer has a stepped portion in accordance with the tapered shape of the first layer.

8. An optical waveguide element according to claim 1, wherein the tip of said second layer has a bent portion that bends toward the tapered edge of said first layer.

9. An optical modulator comprising: the optical waveguide element according to claim 1; a housing for accommodating said optical waveguide element; and an optical fiber for inputting or outputting a light wave to said optical waveguide.

10. An optical modulator according to claim 9, wherein the optical waveguide element is provided with a modulation electrode for modulating the light wave propagating through the optical waveguide, and the housing has an electronic circuit therein for amplifying the modulation signal input to the modulation electrode.

11. An optical transmitter comprising: an optical modulator according to claim 9; a light source for inputting an optical wave to said optical modulator; and an electronic circuit for inputting a modulation signal for modulating said optical wave to said optical modulator.

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